In modern wireless systems, such as for example cellular wireless access and fixed wireless access networks, increasingly high radio frequencies are being used as spectrum becomes scarce and demand for bandwidth increases. Furthermore, antenna systems are becoming increasingly sophisticated, often employing arrays of antenna elements to provide controlled beam shapes and/or MIMO (multiple input multiple output) transmission.
It is known to implement a radio transceiver having an array of antenna elements, where each antenna element may itself be an array of radiator elements. For example, an antenna array assembly for forming controllable beams in azimuth may have a number of antenna elements disposed in an array along a horizontal axis, and each of these antenna element may consist of an array of radiator elements disposed in an array along a vertical axis. Typically, the vertical array of radiator elements may be fed in a fixed phase and amplitude relationship to each other to form a predefined beam in elevation. The amplitude and phase of signals fed to, or received from, each vertical array may be controlled by a beamforming weights matrix to provide controllable beams in azimuth. For example, in a multi-user MIMO (MU-MIMO) system, an antenna array may be used at an access point to form multiple simultaneous beams, each being directed to and/or from a subscriber unit while forming nulls towards other subscriber modules.
There may be radio frequency coupling between antenna elements, which may cause the pattern generated by the antenna array to differ from the pattern that would be expected for an antenna array having high isolation between antenna elements. For example, it may not be straightforward to predict the radiation pattern in azimuth and the maximum radiated power on the basis of weights used to control the amplitude and phase of signals transmitted from antenna elements of the antenna array.
It is an object of the invention to mitigate the problems of the prior art.